The absorption of infrared laser radiation to produce vibrational excitation constitutes, by itself, a well-known method of gas-analysis. The wavelengths of the laser radiation that are absorbed and the amount of absorption are highly characteristic of individual molecular species. They also serve, in devices already on the market, to characterize the state of vibrational and rotational excitation of the absorbing species. Such analytic devices, based on measurement of the amount of infrared radiation absorbed from incident laser radiation, may be termed "infrared (laser) absorption spectrometers". The infrared absorption characteristics of a vast number of molecules have been measured and compiled in connection with these existing devices.
The conventional infrared absorption method of analysis, though highly specific, suffers from the drawback of moderate sensitivity. Such an instrument may, for example, measure approximately 0.1% absorption. This constitutes a small percentage change in the laser intensity which, owing to the high intensity, may, however, represent a substantial amount of infrared absorption, i.e. a large fractional change in the number of vibrationally-excited molecules in the sample gas being illuminated.